This invention concerns a process for the preparation of semicrystalline polycarbonate oligomer compositions from amorphous polycarbonate oligomer compositions in the presence of i) a fugitive plasticizer which acts as a temporary crystallization-rate enhancing agent and/or ii) a particulate nucleating agent. The semicrystalline compositions produced are useful as starting materials for the production of high molecular weight polycarbonate by solid state polymerization.
High molecular weight polycarbonate is a valuable engineering resin useful for producing many objects, especially clear sheeting, compact recording discs and housings for electronic equipment. There are a number of ways this resin can be produced. The most common industrial method is the interfacial polymerization method in which bisphenol-A and phosgene are reacted in a heterogeneous mixture of water and methylene chloride. Although this process produces the desired high molecular weight polymer, there are disadvantages associated with it. Phosgene is extremely toxic and hence results in safety concerns. In addition the use of methylene chloride raises environmental concerns. Finally, the polymer produced by this method contains residues of sodium chloride which are produced by neutralization of sodium hydroxide used to dissolve bisphenol-A in water. This impurity is undesirable in some applications and is difficult to remove. A second method used to produce polycarbonate is the melt polymerization of bisphenol-A and diphenyl carbonate. This process requires the removal of the condensation by-product from the viscous polymer melt. The high temperatures required to achieve low viscosity can lead to degradation of the polycarbonate polymer. A final method known for producing polycarbonate is solid state polymerization. This type of polymerization is widely practiced for the production of polyethylene terephthalate resin for containers. In this process a low or moderate molecular weight polymer is produced and isolated as a solid material such as chips, particles, granules, or powders. Particles of controlled size and shape are most desirable. The polymerization of this solid material is accomplished by heating it to a temperature below its melting temperature with a heated inert gas. The solid state polymerization is thus carried out at lower temperature, which reduces the degradation problem. Before this final step of the solid state polymerization is carried out, the starting materials must be crystallized. For polycarbonate this step is known to be very difficult because of the slow crystallization rate of polycarbonate. Although technologies for crystallization of polycarbonate have been described, all of these technologies have serious drawbacks associated with them.
European Patent No. 0 864 597 discloses a process for the solid state polymerization of polycarbonate oligomer under an atmosphere of a swelling solvent gas or under a stream of a poor solvent gas. The process is applied to either amorphous oligomer particles or powders or to semicrystalline particles or powders. The process does not include a separate crystallization step and hence does not allow one to control the conditions under which crystallization occurs. The swelling solvent gas or poor solvent gas is present throughout the process along with a second inert gas. Since this mixed gas stream will also contain condensation by-products that must be removed during the solid state polymerization, the required constant presence of swelling or poor solvent gas complicates the gas handling requirements of this process, especially if the gas is recycled. Suitable swelling solvents listed include aromatic hydrocarbons, e.g. benzene and substituted benzenes; ethers, e.g. tetrahydrofuran and dioxane; and ketones, e.g. methyl ethyl ketone. Suitable poor solvent gases listed include cyclic hydrocarbons, straight chain or branched saturated hydrocarbons, and unsaturated hydrocarbons.
U.S. Pat. No. 5,191,001 discloses a process for the production of polycarbonate by solid state polymerization of an intimate mixture of oligomeric polycarbonates. The oligomers to be used in this process have a particular endgroup composition. Although crystallization is a required step for this process, the authors do not disclose any particular crystallization technology. A number of general schemes of possible applicability to many polymers are included. The only crystallization method applied is the well-known solution procedure where a semicrystalline powder is prepared by solvent removal from a solution of the oligomers in methylene chloride.
U.S. Pat. No. 5,717,056 discloses a method for preparing a polycarbonate comprising the steps of (a) converting a precursor polycarbonate to an enhanced crystallinity precursor polycarbonate, and (b) polymerizing in the solid state. Converting the precursor polycarbonate to an enhanced crystallinity precursor polycarbonate entails contact at above 110xc2x0 C. with a basic compound. Specific basic compounds listed include alkali metal hydroxides, tetraalkylammonium hydroxides, tetraalkylammonium carboxylates, tetraalkylphosphonium hydroxides, and tetraalkylphosphonium hydroxides. The preferred basic compounds are tetramethylammonium maleate and tetraethylammonium hydroxide. The procedure described to produce this enhanced crystallinity precursor polycarbonate involves contact of polycarbonate particles with a solution containing this basic compound followed by a thermal treatment.
European Patent No. 0 848 030 discloses a process for crystallizing a polycarbonate prepolymer comprising dissolving it in a solvent at elevated temperatures, then cooling the solution to effect crystallization. Preferred solvents are aromatic compounds which form solutions of a concentration of 20-90% polycarbonate. The crystalline product produced is then shaped into the form desired for solid state polymerization. This shape is then dried to volatilize the solvent. This process requires many steps to produce the desired crystallized product.
Japanese Patent Heisei 93 178979 discloses a process for the manufacture of aromatic polycarbonate by solid phase polymerizing crystalline polycarbonate prepolymer characterized in that intermediate polymer that has been solid phased is treated with a crystallization solvent and then subjected again to solid phase polymerization.
It is well known that polycarbonate can be crystallized by exposure to solvents such as acetone. U.S. Pat. No. 5,214,073 discloses a method for preparing a porous crystallized polycarbonate oligomer or prepolymer. In one process described an amorphous polycarbonate oligomer is slurried with acetone to produce the crystallized polycarbonate oligomer. The large amorphous particles that are charged to the acetone bath break up into a very fine powder during the crystallization process. A second process described consists of the melt extrusion of the prepolymer melt into a stirred volume of acetone. This also produces a very fine crystallized powder. Both powders are dried before being subjected to the solid state polymerization. A very fine powder is often not desirable in solid state polymerization because of difficulties associated with material handling.
Provided herein is an improved method for crystallization of polycarbonate or polycarbonate production by solid state polymerization.
This invention provides a process for the preparation of crystalline polycarbonate oligomer compositions from amorphous polycarbonate oligomer compositions comprising the steps of
a) preparing a mixture of the amorphous polycarbonate oligomer with:
i) a fugitive crystallization enhancing agent, and/or
ii) a high melting particulate polymeric nucleating agent
b) forming this mixture into a shape desired, and
c) crystallizing this mixture at a temperature above its glass transition temperature.
Step a) of the above process can be carried out by first producing the oligomeric polycarbonate by contacting a bisphenol with a diaryl carbonate in the melt, in the presence of a suitable catalyst, and then mixing that oligomer with i) a fugitive crystallization enhancing agent, and/or ii) a high melting particulate polymeric nucleating agent. Another method for first producing the oligomeric polycarbonate is by interfacial polymerization of a dihydroxyaromatic compound in the presence of phosgene in solution in the presence of an acid acceptor and an amine as a catalyst. If the fugitive crystallization enhancing agent is used, it can also be mixed with the monomers before oligomerization by either method.
The invention is a method for the crystallization of polycarbonate oligomers which are used in the production of high molecular weight resin by solid state polymerization.
The difficulties in crystallizing polycarbonate prior to solid state polymerization are related to the slow development of crystallinity in this polymer.
The time required to obtain the maximum level of crystallinity in polycarbonate is much longer than for other polymers. The crystallization rate found for polycarbonate oligomer is greater than that of high molecular weight polycarbonate but it is still very low compared to other polymers, such as polyethylene terephthalate, of similar molecular weight, i.e., it exhibits much longer crystallization times. Two factors are the source of the low crystallization rate of polycarbonate. One is related to chain mobility and the high Tg of polycarbonate. Upon cooling from melt, the crystallization rate of polycarbonate increases with decreasing temperature as the driving force for crystal formation, or supercooling, increases. The increase in supercooling is however counteracted by a decrease in mobility as Tg is approached. This understanding leads one to suppose that crystallization rate could be increased by adding a plasticizer that lowers Tg without greatly affecting Tm. This strategy is not desirable since the high Tg of polycarbonate is a very desirable product attribute. This problem is solved by the incorporation of a fugitive crystallization agent. The fugitive crystallization agent is present during operations where it is required, i.e., particle formation and crystallization, but is chosen so that it is sufficiently volatile that it can be removed during solid state polymerization.
The fugitive crystallization agent used in the process of the present invention is described as xe2x80x9cmoderately volatilexe2x80x9d or fugitive in that it can be essentially completely removed during subsequent solid state polymerization of the crystallized oligomer particle by volatilization. Thus, it must have a sufficiently high vapor pressure under solid state polymerization conditions. Crystallization agents with an excessively high vapor pressure are not desirable since they rapidly escape during particle formation from the melt. This leads to problems such as bubble formation, foaming, and contamination of the working environment by the volatile component. The moderately volatile, fugitive crystallization agents usable in the present invention are characterized by a molecular weight greater than 150 g/mole and less than about 600 g/mole. Examples are known plasticizers such as alkyl esters of fatty acids, phthalates, mellitates and materials meeting the above-stated requirements but not normally thought of as plasticizers such as benzophenone and biphenyl. The fugitive crystallization agent is normally present in a concentration of about 5% to about 30% by weight.
The second factor limiting crystallization rate is nucleation. It is generally known that the rate of growth of crystallization can be accelerated in polymers by the addition of a nucleating agent. Examples of commonly used nucleating agents include inorganic oxide materials such as talc, or organic salts such as sodium benzoate. These materials suffer from a common weakness in that they require the addition of a foreign substance, essentially an impurity, to the polycarbonate resin to be produced. In many applications this adversely affects the end use properties of the resin.
Disclosed herein is a process for nucleating the polycarbonate oligomer by the addition of a nucleating agent which is prepared from polycarbonate or other condensation polymer. This invention provides a nucleating agent that is effective in increasing the rate of crystallization but is not a foreign substance since it will ester interchange with the polycarbonate when the polymer is melted after solid state polymerization. An example of such a nucleation agent is a high melting semicrystalline polycarbonate. This material is prepared by extended annealing at high temperature and thus is easily obtained from the product of a solid state polymerization process. This material is either formed into a powder and then annealed or annealed in particle form and then ground to a fine powder, which is used as the nucleating agent.
The method comprises the steps of first forming the appropriate mixture of polycarbonate oligomer with the desired additives. If a fugitive crystallization agent is used, the combination of oligomer plus fugitive crystallization agent may be formed in a number of ways. It is possible to combine oligomer with the fugitive crystallization agent prior to particle formation; e.g., the fugitive crystallization agent is mixed with the oligomer preferably in the melt state or perhaps as a solid prior to particle formation. It is possible to avoid the mixing process by performing the oligomerization in the presence of the fugitive crystallization agent. The fine powder nucleating agent is added to oligomer melt or to a melt consisting of the oligomer and the fugitive crystallization agent. This mixture can then be formed into particles that will have an enhanced crystallization rate. After a mixture of amorphous polycarbonate with either a crystallization enhancing agent and/or a high melting particulate polymeric nucleating agent is prepared it can be formed into a desired shape in several ways. Particle formation can be carried out through a number of processes such as prilling, pastillization and strand cutting. The particle formation and crystallization processes can be carried out as separate steps, for example by quenching the formed particle to an amorphous glassy state and subsequently reheating above the glass transition temperature of the mixture to crystallize, although it is preferred to do these procedures as a single step. This single step process can be done, for example, by using a heated turntable or using a Rotoformer(copyright) pastillator with a heated belt, as disclosed in U.S. Pat. No. 5,633,018. In this technology, a melt or plasticized melt is formed into particles which are held at an appropriate temperature for crystallization, thus combining crystallization and particle formation into a single step. Crystallization occurs at a temperature below the melting temperature and above the Tg of the mixture, generally close to the temperature of maximum crystallization rate. The semicrystalline particles formed can then be solid state polymerized. For example, the particles are heated at 180xc2x0 C. to 250xc2x0 C. under a flow of inert gas or under vacuum to increase molecular weight. The fugitive crystallization agent is chosen so that it can be essentially completely removed during the subsequent solid state polymerization step. The fugitive crystallization agent is initially present during solid state polymerization and may have a second beneficial effect of increasing solid state polymerization rate. Diffusion of the fugitive crystallization agent and volatilization result in a high molecular weight material free of the fugitive crystallization agent and hence having the required high Tg of polycarbonate.
The efficacy of a particular additive can be judged in a number of ways. The rate of crystallization of the polycarbonate oligomer can be characterized by monitoring the change in particle properties when the particle is subjected to the desired crystallization conditions. It is also convenient to characterize the crystallization using standard laboratory tests. DSC, differential scanning calorimetry, is used to determine crystallization rates and crystallization temperatures. This is a widely available technique whose use is well known in the crystallization field. The crystallization temperature is measured by cooling, or heating, at a fixed rate. Heating was done at a rate of about 20xc2x0 C./min and cooling at a rate of about 10xc2x0 C./min. When crystallization occurs, an exothermic peak is detected at some temperature. For materials with very slow crystallization rates no peak is observed on cooling. As the crystallization rate is increased, a peak will appear. The position of this peak is a measure of crystallization rate. On cooling the peak will appear at higher temperature, i.e., with less supercooling, for materials with increased crystallization rate. A similar experiment can be conducted with a material that is first quenched to the glassy state. Upon heating, a crystallization peak occurs and the temperature at which this peak occurs will increase as the crystallization rate is increased.
The molecular weight of polycarbonate oligomers has been measured using the intrinsic viscosity measured by Forced Flow Viscometry using a Viscotek Forced Flow Viscometer Model Y-900. The solvent system used was methylene chloride. For samples with high crystallinity we found that it was necessary to use 50/50 wt % trifluoroacetic acid/methylene chloride. The intrinsic viscosity measured in this mixed solvent can be translated to an intrinsic viscosity measured in methylene chloride by use of a correlation between the intrinsic viscosities in the two solvent systems developed using polycarbonates that are soluble in both systems.